It was only in the 19th and 20th centuries that a broader understanding of the role of different chemical elements arose, providing the scientific foundation for using mineral fertilizers in crop production. Nicolas de Saussure, in 1804 in France, was perhaps the first to show that developing plants require mineral nutrients, insisting that some elements absorbed by plants were indispensable (‘essential’), while others were not. In the 1820s and 1830s, Carl Sprengel in Germany, listed up to 20 elements that he considered to be plant nutrients.
It became clear that identifying the essentiality of specific elements required new techniques. Using nutrient solution culture, Sachs in 1865 identified 10 elements as essential for plants because they were either an integral component of the chemical formula of plant substances (C, O, H, N, S), or because it was demonstrated that the plant cannot complete its vegetation cycle without uptake of any of these elements (P, K, Ca, Mg, Fe). For a long time, it seemed that the list of essential elements would remain at these, but in the 1920s and 1930s, Mn, B, Zn, Cu and Mo were added too.
Terms such as micronutrients or trace elements emerged at that time to depict nutrients that the plant required only in very small amounts for its physiology. The question arose, which of those were indispensable to growth and which were not. Working at the University of California, Daniel Arnon and Perry Stout tackled that issue in this elegant paper by defining three rigid criteria that an element would have to meet in order to be called an ‘essential’ plant nutrient.
One practical implication of this was that a favorable response from adding a given element to the growth medium is not conclusively evident for its indispensability in plant nutrition. The authors were aware of some of the theoretical and experimental limitations of their definitions. They also pointed out that, as scientific methods and knowledge advance, every element in the periodic table might at some point be demonstrated to be essential to plants. Perhaps going far beyond the original intentions Arnon and Stout had, their strict definition of ‘essential’ plant nutrients has prevailed until today. It is still found in many textbooks, and it is also used in standards and regulations for fertilizers worldwide. Their 1939 paper was an important milestone in plant nutrition research, but it is now necessary to replace their definition with one that takes a broader approach to plant nutrition.
